1. Field of the Invention
This invention relates to driver circuits for laser diodes, and more particularly to high-speed, power conditioning driver circuits for laser diodes and laser diode arrays.
2. Background
Laser diodes are continually finding new applications in the commercial, military, medical and other sectors. Laser diodes span the optical spectrum from the near infra-red (IR) through the visible wavelengths, which allows them to be used in a variety of applications, including, inter alia, optical communications, laser pointing and tracking, machining and welding, and pumping of a variety of optically-pumped lasers. Current technology trends all point toward expanded use of laser diodes, especially as efficiency and reliability are improved, and size and operating costs of laser diodes are reduced.
FIG. 1 is a schematic view of a conventional high power laser diode assembly 100 including an array of laser diodes 110. Array 110 includes laser diodes 102 arranged in parallel (rack) and series (stack). A “rack and stack” approach enables the formation of arrays capable of generating high optical power densities (e.g., greater than 1 kW/cm2). Such arrays may require relatively high voltages (typically up to a few kilovolts) and high drive currents (typically up to a few kiloamperes) to operate. Array 110 is mounted on a micro-channel cooling plate 120 to dissipate heat generated by array 110. A one-dimensional or two-dimensional array of laser diodes is referred to herein as a laser diode array (LDA).
While laser diodes have been finding new applications, the breadth of these new applications has been limited by the cost of manufacture, test, and replacement of laser diodes and laser diode arrays.
Common sources of laser diode failure arise from excessive drive currents being provided to a laser diode in an attempt to achieve high laser efficiency (where efficiency is defined as optical power output as a ratio of electrical power input). Exemplary modes of laser diode failure resulting directly or indirectly from excessive drive current include (1) dislocation of and precipitation of host atoms from the laser diode semiconductor crystal, (2) oxidation of the laser diode mirror facets, and (3) metal diffusion of the laser diode electrode and wire bonds.
Controlling the drive current of laser diodes and laser diode arrays (LDAs) to avoid excessive current is complicated by the fact that laser diode junctions are highly nonlinear, dynamic electrical loads, and output optical power can change dramatically with only a small change in input current. One example mechanism of laser diode failure resulting in the modes of failure described above is voltage breakdown of a laser diode's pn junction (also referred to herein as junction breakdown). Junction breakdown occurs when the drive current reaches a critical threshold, which causes strong optical absorption at a crystal defect. This in turn results in localized heating of the crystal, which causes its effective bandgap separation to shrink (and the voltage across a laser diode to decrease), giving rise to further optical absorption and increased drive current. This positive feedback process results in rapid thermal runaway, and breakdown of the pn junction.
Such voltage breakdown is illustrated graphically in FIG. 2, which shows a graphical representation of current versus time beginning with normal diode operation 210, followed by the onset of junction heating 220, during which time current increases and positive feedback begins. Ultimately catastrophic failure 230 occurs if current is not curtailed. Operation in a catastrophic failure regime can result in acute failure of a laser diode. A laser in which current has increased beyond that of normal diode operation is said to be in a “fault state.”
FIG. 3 is a schematic of a conventional power driver circuit 300 having an electrical power source 320 and a semiconductor switch 360 in series with an LDA 310. The pulsing of semiconductor switch 360 is controlled by a switch trigger circuit 365. Semiconductor switches used in conventional driver circuits have included power-field effect transistors (FETs) and integrated gate bipolar transistors (IGBTs).
One drawback of conventional power driver circuits, such as circuit 300 is that the laser diodes (or LDA) powered by the circuits may be exposed to excessive current or current densities in the laser diodes. For example, while switch 360 may limit the duration of excessive current to LDA 310 to prevent catastrophic failure, LDA310 may still be exposed to excessive current in the form of short peaks in current (i.e., transients), which occur over a period of time that is relatively short compared to the duration of pulses from switch 360 or the total current through the diode might constrict within the diode medium and produce local regions of excess current density.
Excessive current or current density may be generated by power source 320, or may be the result of changes in the operating conditions of an LDA such as constriction of the current in the laser diode medium, exposure to electromagnetic fields from other electric devices, electrical breakdown due to ionizing radiation from solar flares, cosmic rays or other sources of electric or magnetic interferences. Additionally, the current-voltage characteristics of an LDA itself may change over the operational lifetime of the LDA.
FIG. 4 is a graphical illustration of an exemplary current waveform 400 of a LDA driven by a conventional drive circuit. In FIG. 4, a semiconductor switch (e.g., switch 360 in FIG. 3) of the LDA driver circuit is turned on at time 410, and turned off 20 microseconds later at time 420. In exemplary waveform 400, during the 10 microsecond period 430 that follows time 420, the LDA is exposed to current transients 435. Even if junction breakdown does not occur, cumulative effect of exposure to such current transients may limit the lifetime of an LDA and cause premature failure.
To reduce the effect of transients and thereby increase the lifetime of LDAs, conventional driver circuits have been operated at reduced average currents and powers; however, reducing the current has resulted in a reduction of the optical output power available from a given LDA assembly, and has limited the applications for which a given LDA may be used.